prostitute ... immoral woman

There are two types of sexually immoral women. The "prostitute" is unmarried, "another man's wife" is married.

a prostitute is a deep pit

"sleeping with a prostitute is like falling into a deep pit"

Here the word refers to any unmarried woman who engages in sexual activity, not only those who do so for money.

These are two places easy to fall into and hard to get out of, the "pit" because it is "deep" and the "well" because it is "narrow."

"Sleeping with another man's wife is like falling into a narrow well"

a hole in the ground that people have dug to get to water